Differential input stages using bipolar junction transistors are inherently non-linear. A simple differential stage shown in FIG. 1 only responds to an input signal voltage lower than 60 mV in magnitude. At higher input voltage levels, one transistor begins to completely turn off while the other conducts all the current. The resulting output current effectively reaches a limit for input voltages in excess of 60 mV. The input voltage to output current transfer function for the circuit of FIG. 1 is shown in FIG. 2. The input signal often needs to be much lower than 60 mV. When signal distortion is critical, the input peak voltage must be kept below 17 mV for the distortion to be kept below 1%. This is a factor of ten lower in voltage than the 200 mV level commonly is used in audio applications.
One method to increase the maximum allowable input signal range was proposed by Okanobu in U.S. Pat. No. 4,965,528 issued Oct. 23, 1990. In this invention which is shown in FIG. 3, the outputs collectors of two equally biased differential input stages which both have opposing N:1 area ratios are summed together. FIG. 4 shows that the resulting input voltage range on the transfer curve is effectively doubled above that of a two transistor differential stage shown in FIG. 2. There is also a reduction in distortion due to a limited amount of non-linearity cancellation between the two stages. This lowering of distortion further to extend the useful input voltage range. This input stage is not Class-AB biased since the supply current remains constant as a regardless of input voltage.